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        /* 
            条件分支语句，也叫switch语句
            语法：
            switch(条件表达式){
                case 表达式:
                语句....
                break;
                case 表达式:
                语句....
                break;
                case 表达式:
                语句....
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                case 表达式:
                语句....
                break;
                defaut:
                语句....
                break;
            }
         */
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